This invention relates to a motion picture film incremental drive for a motion picture projector and more specifically to motion picture projection of 16 mm (millimeter) film.
Historically, 16 mm film with sprocket holes has been pulled down intermittently by means of a claw or finger type reciprocating mechanism (Geneva type). With the advent of high-speed motors, which can be stopped and started very quickly and can be electronically controlled, a much faster and more efficient prime moving film system becomes feasible. The basic problem precluding the use of this type of system is the vulnerability of the sprocket holes in the film.
Normally, only two sprocket holes in 16 mm film are engaged by the film pull-down system. In a fast pull-down or high-speed system, torque is used to move the film. If only two holes are engaged, the uniformity of the sprocket holes in the film may be destroyed. The faster speed available from servo type motors magnifies the problem.
In addition there is the problem of centering the film frame in the gate aperture before illumination. Precise centering in 35 mm has been done by measuring optically the passage of sprocket holes. While for 35 mm there is 4 sprocket holes per frame, there is only one sprocket hole per frame in 16 mm. Also the start and stopping of a 35 mm projector using a servo motor is dependent upon a changing D.C. bias control by counting the sprocket holes. For a 16 mm projector using such a servo motor there is only one hole per frame to control the servo motor.